Remove hardcoded textbox colours

We have a custom textbox colour system, why not use it? This also moves
the map of colours from CustomLevels to Script.

desktop_version/src/CustomLevels.h

@@ -170,8 +170,6 @@ public:
     SDL_Color getonewaycol(int rx, int ry);
     SDL_Color getonewaycol(void);
     bool onewaycol_override;
-
-    std::map<std::string, SDL_Color> customcolours;
 };
 
 std::string translate_title(const std::string& title, bool* is_gettext);